When the exact folium setup can't be identified, it is advised to use <folium> elements only (and not guess about the presence of <bifolium>s in the document).
<elementSpec ident="folium" module="MEI.header">
<desc xml:lang="en">Describes a single leaf of paper.</desc>
<classes>
<memberOf key="att.common"/>
<memberOf key="att.dimensions"/>
<memberOf key="att.measurement"/>
<memberOf key="att.foliumSurfaces"/>
<memberOf key="model.foliumLike"/>
</classes>
<remarks xml:lang="en">
<p>When the exact folium setup can't be identified, it is advised to use <gi scheme="MEI">folium</gi> elements only (and not guess about the presence of <gi scheme="MEI">bifolium</gi>s in the document).</p>
</remarks>
</elementSpec>